Acrosome: Reaction, Function & Definition
When a sperm finds an egg, the complex process of fertilization has only just begun. In this lesson you will learn about the acrosome, a specialized vesicle in the head of the sperm, and the important roles it plays in fertilization.
A Long Journey
Let's be honest: sperm have a hard job to do. First, they have to actually find an egg to fertilize. Even after swimming long distances through treacherous conditions, the vast majority of these tiny, short-lived cells do not find an egg. And those few sperm that do find an egg are not yet finished with their work; they still have to cross the egg's protective coat and fuse their own membrane with the plasma membrane of the egg. In almost all cases, only one sperm out of the millions released manages to do this. Its nucleus and genetic material enter the egg, and the development of a new organism begins.
The Anatomy of a Sperm
To understand the acrosome reaction, we first need to understand the different components involved. Here's a basic diagram of the anatomy of a sperm. The tail is, of course, for swimming, and the head contains a nucleus, a pool of actin and an acrosome. So what are these parts for?
The acrosome is a vesicle positioned close to the plasma membrane at the tip of the sperm's head. This vesicle contains soluble proteolytic enzymes (depicted in the diagram as pink contents) and inner membrane proteins such as bindin (depicted as cyan-colored dots).
The nucleus (depicted in green) contains the genetic material that the sperm is trying to pass on, a haploid genome because it contains only one copy of each chromosome.
In front of the nucleus and behind the acrosome, the sperm stores actin (depicted in purple). Actin is a cytoskeletal protein that exists in two major forms in cells: globular or G-actin and filamentous or F-actin. G-actin proteins are monomers that can polymerize into long F-actin filaments that can change the shape of the cell. As we will see, this is important during the acrosome reaction.
The Acrosome Reaction
The acrosome reaction is the exocytosis of the acrosome, or the fusion of the acrosomal membrane with the sperm's plasma membrane. This process releases the acrosomal contents to the outside of the sperm, and exposes the inner acrosomal membrane proteins on the sperm's outer plasma membrane.
The function of the acrosome reaction is to help the sperm get through the egg's protective coat, and to allow the plasma membranes of the sperm and egg to fuse. This places both haploid nuclei (one from the sperm and one from the egg) into the same cell, where they form the diploid genome of the new organism.
The acrosome reaction is diagrammed below. The reaction is initiated by the binding of the sperm to molecules in the egg's protective coat. This binding causes a calcium influx into the sperm's cytosol, which stimulates the exocytosis of the acrosome. This has three important consequences.
Release of Proteolytic Enzymes
When the acrosome's proteolytic enzymes are released, they begin to degrade the egg's protein-rich protective coat. This creates a path for the sperm to swim through on its way to the egg's plasma membrane.
What is this protective egg coat we keep referring to? It's different in mammals than in marine organisms such as sea urchins, whose external fertilization process has often been studied. In sea urchins, the egg plasma membrane is surrounded by a thin, porous vitelline envelope made of extracellular matrix, and then a thicker jelly coat surrounding that. In mammals, the egg plasma membrane is surrounded by a thick zona pellucida layer, made of extracellular matrix, and a jelly-like cumulus layer surrounding that.
In both marine organisms and in mammals, the egg's coat protects the egg from damage and helps to prevent cross-species fertilization, as that could result in inviable offspring. In a marine environment, this is especially important because there are lots of eggs and sperm in the water that have all been released from various different organisms. The egg's coat also plays a role in preventing polyspermy, or fertilization by more than one sperm, which would also result in inviable offspring.
Extension of the Acrosomal Process
The G-actin in the sperm's cytosol polymerizes into F-actin, creating an extended projection at the tip of the sperm's head. This pointy projection is called the acrosomal process and it is the part of the sperm membrane that is going to fuse with the egg's plasma membrane.
Exposure of Membrane Proteins
Because of the exocytosis of the acrosome, membrane proteins that were on the inner acrosomal membrane are exposed on the outside of the sperm's plasma membrane, surrounding the acrosomal process. Some of these membrane proteins help the sperm stay bound to the egg's protective coat so that it keeps burrowing through.
Once the sperm has gotten through the protective coat and reached the plasma membrane, other membrane proteins, such as bindin, bind to receptors on the egg's plasma membrane. This binding triggers the fusion of the egg and sperm plasma membranes. Sperm bindin and egg bindin receptors are species-specific: they will only interact properly if both egg and sperm are from the same species, so this interaction is another block to cross-species fertilization.
To review, here is a diagram of the acrosomal reaction in the context of a sea urchin egg that is being fertilized.
When the sperm binds to the egg's protective coat, the acrosome is exocytosed. The proteolytic enzymes from inside the acrosome digest the proteins in the egg's coat. The acrosomal process forms when G-actin polymerizes into F-actin filaments. The acrosomal membrane proteins help the sperm stay bound to the egg coat as it burrows through, and also help it bind and fuse with the egg's plasma membrane once it gets there. Once the plasma membranes are fused, the sperm's nucleus enters the egg cytosol and the sperm's work is done at last!
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